Petrogenesis and tectonic significance of Late Mesozoic A-type granite from the Huangmeijian intrusion in the Luzong volcanic basin, eastern China: Constraints from geochronology and geochemistry

Abstract

The Late Mesozoic A-type granites are widely distributed along eastern China's Lower Yangtze River Belt (LYRB). Much attention has been paid to this region's adakitic rocks and ore deposits, whereas few studies have examined A-type granites. The largest A-type granitic pluton within the Luzong basin, the Huangmeijian (HMJ) intrusion, is composed of quartz syenite, syenogranite, syenite porphyry, and a minor quartz monzonite. Syenite porphyry in the HMJ is divided into fine-grained syenite porphyry (FSP) and coarse-grained syenite porphyry (CSP) with different grain sizes, but studies on syenite porphyry are sparse. In this study, we reported a detailed study including the in situ U–Pb age and trace-element data, whole-rock major and trace elements, apatite compositions, and Sr-Nd isotopes from the HMJ pluton, with aims to further constraints on the petrogenesis, magmatic sources, and evolution, and tectonic significance of A-type granites in the LYRB. These two syenite porphyries (FSP and CSP) in HMJ intrusion were delineated by zircon U–Pb dating with the ages of 126.4 ± 4.4 Ma and 128.9 ± 0.54 Ma respectively. They have geochemical characteristics similar to those of A-type granites, with high total-alkali (10.5–10.7 wt%), HFSEs (Zr + Nb + Ce + Y = 540.4–949.4 ppm), high Ga/Al ratios (10,000 × Ga/Al = 3.88–4.57), and high whole-rock zircon saturation temperatures (832–906 °C) and high oxygen fugacity, classified as A1 type granite. The slightly enriched Sr-Nd isotope composition (87Sr/86Sr = 0.70428–0.71275, εNd(t) = −3.26 to −5.86) of the HMJ apatite with high F and Cl contents, coupled with trace element features, indicating that syenite porphyry was likely originated from metasomatized mantle-derived magma. In addition, two groups of ancient inherited zircons were observed in the FSP, implying the contribution of the old crystalline basement and Paleozoic materials in the magma source. Moreover, significant hostile Eu, Sr, and Ba anomalies and depletion of Sr and Eu in apatite suggest the fractionation of plagioclase, K-feldspar, apatite, and Fe-Ti oxides are also reflected in their elemental variations. Comprehensive discriminant analysis shows that the HMJ syenite porphyries were derived from the metasomatized mantle-derived magma mixing with ancient crustal components in the Paleo-Pacific plate roll-back and intensive lithospheric extension setting, and followed by fractional crystallization.